June 14, 2010:
Revised: v2.0

ZL2PD Fixed Switchmode Power Supply

This page describes how to modify a surplus switchmode phone charger to create a lightweight fixed voltage output power supply for other uses.


This old cellular phone charger was originally made by Salcom Oy in Finland for an Ericsson GSM cellualr phone. I used it to build a lightweight AC power supply to power a microprocessor emulator. Other types of chargers and power supplies can easily be changed to suit a variety of applications by following the details described below.

The Beginning        

 As I've mentioned elsewhere on this website, I travel quite a bit in connection with my work. A day or so before one of my trips, I suddenly decided I just had to take my Philips 8051 microprocessor emulator with me. I had been working on another microprocessor based project in my spare time, and it felt close to completion. A long gap away from the project would have really set me back. With some free weekends in some fairly remote places coming up, I reckoned that if I took the emulator with me, I could finish the project and make better use of my time.

These sudden whims of mine can have unexpected results. Fortunately, this time, the outcome was positive. It resulted in the construction of a lightweight space saving power supply for my emulator, and, as a result, the completion of that software. This project is also a good example of how I modify small switchmode power supplies to suit specific applications.    

The Problem    

My 8051 emulator is quite compact. But as I was packing it for my trip, I suddenly realised that the emulator's power supply was going to be a bit of a problem. It's an old transformer powerpack, rated for continuous operation at 9V and 500mA. I originally built it perhaps ten years ago, adding a switchmode single IC regulator to a simple transformer-rectifier power pack.

This power pack was heavy. Really heavy. Normally it's no problem. I just use it around my office, with the emulator plugged into my laptop or PC. (See the photo to the left) But for a long trip, the power supply's weight could make it impractical to take the emulator. It weighted in at almost 0.8 kg! (Just under 2 lbs for those disadvantaged folk still living in the non-metric world) No way was I going to carry that around in my suitcase.

(Those that know my travel habits will smile - Here I was worrying about an extra kilo when my bag often weighs in at 20++kg at the check-in counter. But my motto is - A kilo saved from one item is an excuse to carry another kilo of something else….)          

The Solution!    

Searching around in my cardboard box of salvaged power packs, I found an old Ericsson celphone charger with a rating of 7.6V/600mA which looked like it would be ideal for the task. But the output voltage of 7.6VDC was too low for the emulator. My emulator needs at least 8.5V into the on-board regulators to give satisfactory operation.

Having determined that it was a switchmode power supply, confirmed by the charger's label, the next step was to open the charger to see if the circuit was suitable for the planned modification. I don't bother if the power supply lacks an optocoupler or if it is poorly constructed with components which appear to have been dredged from the bottom of a river. This is something I frequently see in chargers on cordless phones with odd-sounding brand-names. In this case, the charger was a high quality design with a clean layout.

Opening the charger case was arguably the most difficult part of the whole project. It really took quite a bit of force to open it. Unlike an earlier Ericsson charger which I modified several years before, one with an identical part number which was simply screwed together, the two halves in this unit had been glued together in the factory, and this took quite a bit of effort with a pair of screwdrivers to gradually prise the two halves of the case apart along the plastic seams.

The photo at the top of the page shows the power supply just after I'd opened it up.    


(Sorry - I just have to add this note here to keep those salivating lawyers at bay)

Do not attempt the modifications described here unless you fully understand the risks associated with mains voltage switch mode power supplies and you are confident in your ability to complete any evaluation, measurements and changes safely.

Circuit voltages can easily rise above 300VDC, and, under some circumstances, to considerably higher voltages, especially if modifications are not carried out correctly. Voltages and currents present in these circuits, both before and after modification, can therefore pose potentially lethal risks to the unwary.

Do not attempt to copy these designs or to modify your own switchmode AC power supplies unless you have adequate expertise and experience with such circuitry. Furthermore, you should only attempt these modifications if you can, firstly, accurately analyse the specific power supply you are planning to modify.

In short, don't come complaining to me if you kill yourself while attempting to modify one of these power supplies!!
The charger's internal layout is very tidy and key components can be readily identified. (Right click on the photo at the left here to see the internals of this power supply) This allowed the appropriate modifications to be determined easily and the required changes to be made rapidly.    

Once opened, the design was found to be completely different from a previous Ericsson charger I had modified for another use. I quickly determined this latest one used a TL431 as the voltage detector, with the output voltage set by two resistors. (See the diagram and photo below) Feedback to the primary side was via an optocoupler.

And, within a little over ten minutes, I had completed the modification. All that was required was a extra 100k resistor in parallel with R2 to give an output voltage of exactly 9V. The photo below shows the resistor mounted on the underside of the PCB. I used a wooden toothpick as a pointer in the photo.

I also removed the original DC output connector and replaced it with a short DC cable with an emulator-compatible DC connector. I clamped the new cable in place to the PCB with a nylon cable tie using the old DC connector's mounting holes.        

Typical output voltage detection system using a TL431 shunt regulator uses a two resistor voltage divider (R1 and R2)        

A simple single resistor modification allowed this power supply to be converted to the required output voltage of 9V. The added resistor - the toothpick is pointing to the resistor -  is at the upper left hand corner of the PCB

Experimenting with resistor values on an open power supply with mains AC voltage applied is key to determining the appropriate values to use. But it is potentially a very dangerous procedure. I used a variable preset resistor and a couple of fixed resistors to find the correct component value to use in the final modification, with the components roughly soldered to the back of the PCB.

To make it safer, I wedged the board in a short length of plastic square section tube during these tests, and covered as much of the board as I could to avoid any accidental contact with the dangerous and potentially lethal voltages present in the power supply. The preset resistor was adjusted with a ceramic screwdriver.

I must stress the need to take great care with this part of the process if you choose to try this for yourself. Read the other warnings on this page too before you try these modifications.

Before beginning any modifications, I also took a really careful look at the circuit board layout. I noticed that the unmodified power supply output was protected against overvoltage by a PCB fuse and a 12VDC zener. Since the planned modification would also give an output voltage below 12V,  this arrangement could be left unchanged. The unit also featured an extra SMD LM334 IC constant current regulator circuit fed from the output voltage rail which then fed a 'Power On' green LED indicator. A quick check with the LM334 datasheet before making any modifications determined that this circuit would work fine with the increased output voltage.

After testing, the power supply case was reassembled with rapid setting glue ('Superglue'). I removed the old Ericsson label and replaced it with a new label detailing the revised power supply performance. This paper label was glued in place and covered with heavy duty clear self-adhesive plastic film.

The Results    

The output of the power supply with this modification delivered exactly 9.0VDC. Under full load (500mA), the output voltage fell by only 30mV, a better performance than the original transformer power supply. Also, the power supply remained very cool, indicating excellent efficiency.

The new power supply was put on the scales just to see how successful I had been in reducing the power supply weight. The new switchmode power supply weighed in at 100 grams! (3.5 oz) That's about 1/8th of the weight of the original unit. Size-wise, it occupies a similar volume, but its shape made it much easier to pack in the same box with the emulator. So I count it as a successful space-saving modification too, I think.

The power supply has worked very well during my lengthy travels. I even finished the microprocessor project!    

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